Re-Purposing a Motorola Mocom 70 Control Head for Modern Use
with a Maxtrac Mobile
By Jerry Coffman, K5JC
Abstract
Vintage radio control heads often featured robust mechanical construction and intuitive operator controls.
This project demonstrates how a classic land-mobile Motorola Mocom 70 police-style control head can be
interfaced with a later-generation Motorola Maxtrac mobile radio using the 16-pin accessory connector. By
re-purposing existing switches and controls, the Mocom head becomes a functional remote interface
providing channel selection, transmit control, and audio handling. Included will be step-by-step
construction, testing and troubleshooting.
Background and Equipment
The Motorola Mocom 70 control head is a rugged, mechanically simple interface widely used in public
safety service during the 1960s and 1970s. These early models employed discrete wiring and switch logic
for control functions. In contrast, later radios such as the Motorola Maxtrac use microprocessor control
with programmable I/O via an accessory connector. The Motorola Maxtrac provides synthesized
operation and a flexible accessory connector capable of external control inputs. Combining these two
allows vintage controls to operate a modern multi-channel radio.
A useful feature of the Maxtrac is its support for external channel steering, allowing channel selection
through binary logic inputs.
Why Combine a Mocom 70 with a Maxtrac?
This hybrid system combines the durability and ergonomics of vintage hardware with the flexibility of
synthesized radios. It enables authentic installations in vintage vehicles, eliminates the need for crystals,
provides up to 16 channels, and offers reliable operation using simple ground-switch logic.
Figure 1 — Mocom 70 control head.
Figure 2 — Motorola Maxtrac radio.
Figure 3. Connection Summary.
System Overview
The conversion uses the
Mocom 70 control head as a passive control interface, providing:
Push-to-talk (PTT)
Microphone audio
Volume and squelch control with mods
Channel selection via toggle switches
External speaker audio
The Maxtrac provides all RF and audio processing functions.
All control heads do not have all switches necessary for all four channels, obviously a four channel control
head has more flexibility than one with only one optional switch (for example NB, PL, MON)
Complete Step-by-Step Construction
Step 1 — Identify Ground: Locate the black wire and verify continuity to chassis.
Step 2 — Verify Switches: Ensure NB, PL, MON, HOOK provide ground when ON and open when OFF.
Step 3 — Identify Mic/PTT: Locate microphone audio and PTT leads; verify PTT grounds when pressed.
Step 4 — Ground Connection: Connect Mocom ground to Maxtrac pin 7.
Step 5 — Channel Wiring: Connect switches to pins 6, 8, 9, 12 for binary channel selection.
Step 6 — PTT Wiring: Connect PTT to pin 3.
Step 7 — Mic Audio: Connect mic audio to pin 2 with shield grounded at one end.
Step 8 — Speaker: Connect speaker to pins 1 and 16 (floating, do not ground).
Step 9 — Programming: Configure Maxtrac RSS for external channel steering (active LOW).
Step 10 — Pre-Power Check: Verify no shorts and no voltage on control lines.
Step 11 — Power On: Apply power and confirm normal startup.
Step 12 — Channel Test: Verify switches change channels correctly.
Step 13 — PTT Test: Confirm transmit operation.
Step 14 — Audio Test: Verify clear transmit and receive audio.
Volume and Squelch
The Mocom 70 uses 10 kΩ potentiometers for volume and squelch:
Volume control may be inserted in the speaker audio line for trimming (optional)
Squelch control is optional and may be omitted or repurposed
In most installations, the Maxtrac’s internal squelch is sufficient.
Electrical Interface and Wiring
Figure 1 shows the complete interconnection between the Mocom 70 control head and the Maxtrac 16-pin
accessory connector.
Figure 1 — Complete wiring diagram of the Mocom 70 control head interfaced to a Motorola
Maxtrac. Shown are microphone audio, PTT, channel steering lines, and speaker connections. All
switch inputs are ground-activated (active low).
Maxtrac Pin
Function
Mocom Connection
2
Mic Audio Input
Mic high (WHT)
3
PTT (active low)
PTT switch (RED)
6
Channel Select 1
NB switch
8
Channel Select 2
PL switch
9
Channel Select 3
MON switch
12
Channel Select 4
Hook switch
7
Ground
Common (BLK)
1 & 16
Speaker Output
Mocom speaker
Important: The speaker output is floating. Do not ground either lead
RSS Programming—Using Motorola RSS
Menu Path:
F4 (Change/View) → F2 (Radio Wide) → F9 (Accessory Configuration)
Pin
Assignment
6
Channel Select 1
8
Channel Select 2
9
Channel Select 3
12
Channel Select 4
Configuration:
External Channel Steering
Active Low Inputs
Debounce Enabled
Speaker Audio Integration
Audio from the Maxtrac is routed directly to the Mocom speaker:
Pin 1 → Speaker return
Pin 16 → Speaker high
This preserves the original control head acoustics
Max Trac Pin functions
Pin Function Notes
1 External Speaker – Floating output
2 Mic Audio Programmable input
3 PTT Push-to-talk input
4 Programmable I/O User programmable
5 Flat TX Audio For repeaters/data
6 Programmable I/O Channel Select 1 (your project)
7 Ground Common return
8 Programmable I/O Channel Select 2
9 Programmable I/O Channel Select 3 (often ships as Emergency; reprogram if needed)
10 Ignition Sense +12 V ignition input
11 Flat RX Audio (Discriminator) Receiver audio output
12 Programmable I/O TX PL Inhibit
13 Switched +13.8 V Low-current output
14 Programmable I/O Monitor (recommended for your Extender switch)
15 Internal Speaker Enable Jumper to pin 16 if using internal speaker
16 Speaker Audio Floating speaker outp
Mocom 70 4-Channel Head → MaxTrac Channel Steering
Control Pin MaxTrac connection
Channel 1 7 Steering matrix → pin 6
Channel 2 9 Steering matrix → pin 8
Channel 3 8 Steering matrix → pins 6 + 8 diode connecting them
Channel 4 10 Steering matrix → pin 9
PL switch 16
Extender 18
Ground 4 MaxTrac pin 7
Mocom 70 2-Channel Head → MaxTrac Channel Steering
Mocom 70 TB501 Pin Function Used MaxTrac Channel MaxTrac Pin(s)
7 Channel 1 Channel 1 Pin 6 (Bit 1)
9 Channel 2 Channel 2 Pin 8 (Bit 2)
18 Extender Channel 3 Pins 6 + 8 (Bits 1 + 2)
diode connecting them
16 PL Switch Channel 4 Pin 9 (Bit 3)
Mic audio 2 MaxTrac pin 2
PTT 3 MaxTrac pin 3
Ground Common — Pin 7
MaxTrac Channel Steering Pins
MaxTrac Function Pin
Bit 1 6
Bit 2 8
Bit 3 9
Ground 7
Use 1N4148 diode. Between pins 6 & 8 Other wise, 6 and 8 will be connected permanently
Channel Steering Concept
The Maxtrac supports up to 16 channels via four binary inputs. Each input is internally pulled high and
becomes active when grounded. Figure 2 illustrates the binary relationship between switch positions and
selected channel.
Figure 2 — Binary channel steering using four Mocom control switches. Each switch represents one
bit (1, 2, 4, 8), allowing selection of up to 16 channels
Channel
NB (1)
PL (2)
MON (4)
HOOK (8)
1
0
0
0
0
2
1
0
0
0
3
0
1
0
0
4
1
1
0
0
5
0
0
1
0
6
1
0
1
0
7
0
1
1
0
8
1
1
1
0
9-16
Add HOOK = 1
Bench Wiring Checklist
Check
Status
Ground verified
Switches verified (ground closure)
No voltage on control lines
PTT operational
Mic audio clean
Channel switching correct
Speaker isolated
No overhearing
Troubleshooting
• No transmit: check PTT wiring (pin 3)
• No audio: check speaker pins 1 and 16
• No channel change: verify pins 6, 8, 9, 12
• Hum/noise: correct grounding and shielding
Parts List
Item
Description
1
Mocom 70 control head and speaker
2
Motorola Maxtrac radio
3
16-pin accessory connector
4
Shielded audio cable
5
Hook switch (optional)
Example 6-Meter Channels
Channel
Frequency
1
52.525 MHz
2
52.540 MHz
3
52.570 MHz
4
52.600 MHz
Construction Notes
Verify all switches are ground closures only
Avoid lamp or 12 V circuits
Use shielded cable for audio
Tie all grounds to Pin 7
Optional: add transistor buffering for additional protection
Bench Testing Procedure
1. Disconnect power
2. Verify wiring continuity
3. Confirm no voltage on logic lines
4. Apply power
5. Test PTT
6. Verify mic audio
7. Test channel switching
8. Confirm speaker audio
9. Adjust levels
Results and Discussion
The completed system combines the rugged mechanical interface of a vintage control head with the
flexibility of a synthesized radio. Channel selection via toggle switches provides a simple and reliable user
interface
Conclusion
The Motorola Mocom 70 control head can be successfully repurposed as a remote interface for a Maxtrac
mobile. This project highlights the continued usefulness of legacy hardware in modern amateur radio
applications
Author Information
Jerry Coffman K5JC is an amateur radio enthusiast with a long-standing interest in vintage land mobile
radio equipment and practical conversion techniques for amateur use.
